IEEE802.11 provides a cost-effective solution for networking terminal apparatuses including computers, for example, by wireless. With new developments in signal processing and modulation technologies, enhancements have been made in standards supporting new physical layers with higher data rates. Studies have shown that the key limiting factor in current 802.11 system is the MAC (Medium Access Control) layer, where throughput saturates with increase in data rates (see, for example, non-patent document 1). The IEEE802.11 working group has identified the need for a high-throughput wireless LAN configured based on both MAC and PHY changes to existing wireless LANs.
For current applications and those envisioned for the future, the data rates supported by existing wireless LANs are sufficient. Wireless LANs have heretofore employed time division multiple access schemes, and the problem with this lies in the number of users of high data rate applications that the network can support at a time. The problem can be generalized as one of the need for higher throughput. To achieve higher throughput, it is necessary to improve data rates measured in layers above layer 2, or the medium access control (MAC) layer, in the open system interconnection (OSI) by the International Organization for Standardization (ISO). To meet the requirement of increasing throughput for all wireless terminal apparatuses in a typical wireless LAN system containing a single wireless base station apparatus (for example, an access point (AP)) and a plurality of wireless terminal apparatuses (STAs), throughput is measured at the AP.
Communication in a conventional wireless LAN system is based on the CSMA scheme. That is, whether or not an STA is able to transmit a data packet is determined by detecting whether or not a medium to be accessed is busy (occupied) or idle (unoccupied). One approach to improve throughput in this wireless LAN system is to exploit the benefits of the space division multiple access (SDMA) scheme. However, optimum scheduling is necessary for this purpose, such as, for example, performing transmission and reception with different STAs with antennas of different APs.
Non-patent Document 1: “Throughput Analysis for IEEE 802.11a Higher Data Rates”, doc.: IEEE 802.11/02-138r0, March 2002.